This invention is directed to a switching device for selectively actuating or deactuating the stereo expansion circuit of an audio system.
Stereo expansion in audio systems is well known in the art and has been available for many years. In such systems, the right and left channel signals are processed in a manner which makes it appear to the listener that the separation of the speakers is much greater than the actual physical separation. In the prior art, stereophonic systems having expansion capability typically include a relatively complex electronic switching arrangement which enables the incoming signals to be input either directly to the audio processing system when expansion is not utilized or through a stereo expansion circuit when expansion is utilized. This selection circuitry and switching circuitry therefore substantially adds to the complexity and cost of the stereo receiver. Another difficulty with the prior art expansion system arises because the expansion circuitry changes the impedance from the incoming circuitry. For this reason, impedance matching devices must be added to the expansion circuitry in order to optimize the operation of the processing circuitry.
FIG. 1 is a simplified block diagram showing the selection of expanded stero in a prior art system. The right (R) and left (L) audio signals are received on input lines 1 and 1a respectively. A selection switch 2 is shown as a double-pole double-throw switch for purposes of illustration, in an actual stereo receiver the switch 2 is an electronic switch, many types of which are available. When expanded stereo is not desired the switch is coupled to lines 1 and 1a and the R and L signals pass directly through the switch 3 to output lines 8 and 9. The input lines 1 and 1a are also coupled to a stereo expansion circuit 3 having output lines 5 and 5a. The expansion circuit 3 processes the R and L audio signals in known manner and the expanded audio signals L+X(L--R) and R+X(R--L) are available on output lines 4 and 5 of the expansion circuit 3. An impedance matching circuit 6 is arranged between the expansion circuit 3 and the switch 2. The impedance matching circuit is used to match the impedance of the expansion circuit 3 to that of the subsequent circuitry. When stereo expansion is desired the switch 2 is connected to the output lines 7 and 7a of the impedance matching circuit 6 and the L+X(L--R) and R+X(R--L) signals are available on the output lines 8 and 9 respectively of the switch 2. Accordingly, in the prior art the impedance matching circuit 6 and the switch 2 are needed to provide for selective stereo expansion capability. In highly competitive markets, such as consumer electronic markets including stereophonic color television receivers, savings in cost and complexity are of paramount importance. For these reasons there is a need for a simple, inexpensive but effective stereo expansion circuit selection switch. The present invention fulfills these needs.